Diglycerides are used as an additive for improving plasticity of oils or fats or in the fields of foods, pharmaceuticals, cosmetics, etc. Foods utilizing the excellent physiological activity of diglycerides have recently drawn attentions.
As a typical production process of diglycerides, esterification or ester exchange reaction can be given. For example, a process of obtaining diglycerides by reacting a fatty acid or a lower alkyl ester thereof with glycerin in the presence of immobilized 1,3-specific lipase and taking the resulting water or lower alcohol, which is generated upon the reaction, out of the system by pressure reduction is known (Japanese Patent Publication No. Hei 6-65311).
Since not only diglycerides but also triglycerides are prepared by the esterification reaction using 1,3-specific lipase, a rise in the reaction yield decreases the purity of diglycerides. Decreasing in the reaction yield, on the other hand, increases the purity of diglycerides, but a large amount of glycerin or fatty acid remains unreacted. In order to increase the purity of diglycerides, a purification step such as stripping is therefore inevitable after esterification reaction.
As a method for overcoming the above-described problem, proposed is a process for preparing triglyceride-free glycerides by using partial glyceride lipase which does not act on triglycerides (Japanese Patent Application Laid-Open No. Sho 61-181390). This method is however not suited for the production of a high purity of diglycerides, because monoglycerides are main products of this method.
In the conventional esterification reaction which has been employed industrially, it is necessary to remove, out of the system, water generated with the progress of the reaction, and shift the reaction equilibrium, thereby increasing the reaction yield. The esterification reaction using a partial glyceride lipase while removing water out of the system under reduced pressure involves problems such as termination of the esterification reaction before termination. Also proposed is a process (Japanese Patent Application Laid-Open No. Hei 1-137989), in the synthesis of triglycerides by acting a partial glyceride lipase on an oil or fat containing partial glycerides and fatty acids, of carrying out the esterification reaction under a low water content while using an immobilized enzyme. This process is for the synthesis of triglycerides while suppressing the yields of mono- and diglycerides, and is essentially different from the process for preparing a high purity of diglycerides.
When diglycerides are synthesized by the esterification reaction, the optimum terminal point of the reaction for attaining a high diglyceride purity at a high reaction yield, depending on the substrate ratio, as raw materials, of a fatty acid or ester thereof to glycerin, reaction temperature, enzyme concentration or the like. Particularly when a ratio of the fatty acid or ester thereof to glycerin is raised for improving a reaction rate, an increase in a reaction yield by prolonging the reaction leads to a problem such as decreasing in a diglycerides purity due to the formation of triglycerides.